In optical communications using optical fibers for the communication transmission path, together with progress of the technology used in its utilization and expansion of the range of utilization, there is a need for increasing distance of the communication transmission path and increasing speed of the communication bit rate. In such an environment, the dispersion that occurs in a signal light transmitting through optical fibers becomes a serious problem, and various attempts have been made to compensate for that dispersion. At the present time, the second order dispersion has become a serious problem, and various proposals have been made for its compensation, several of which have been effective to a certain extent.
However, as the demands being placed on optical communications become increasingly severe, compensation of the second order dispersion only during transmission has become insufficient, and compensation of the third order dispersion is becoming an important topic.
The following provides an explanation of a conventional method of compensating for the second order dispersion using FIGS. 7A through 7C and FIG. 8.
FIG. 8 is a drawing that explains the dispersion vs. waveform characteristics of a single mode optical fiber (hereinafter, also to be referred to as SMF), dispersion compensating fiber and dispersion shift fiber (hereinafter, also to be referred to as DSF). In FIG. 8, reference symbol 601 indicates a graph of the dispersion vs. wavelength characteristics of an SMF, reference symbol 602 indicates a graph of the dispersion vs. wavelength characteristics of a dispersion compensating fiber, and reference symbol 603 indicates a graph of the dispersion vs. wavelength characteristics of a DSF. In the graphs, dispersion is plotted on the vertical axis and wavelength is plotted on the horizontal axis.
As is clear in FIG. 8, in the SMF, as the wavelength of the light that is input to the fiber becomes longer from 1.3 μm to 1.8 μm, dispersion increases, while in the dispersion compensating fiber, as the wavelength of the input light becomes longer from 1.3 μm to 1.7 μm, dispersion decreases. In the DSF, as the wavelength of the input light becomes longer from 1.2 μm to around 1.55 μm, dispersion decreases, and as the wavelength of the input light increases from around 1.55 μm to 1.8 μm, dispersion increases. In the DSF, in optical communications at conventional communication bit rate on the order of 2.5 Gbps (2.5 gigabits per second), dispersion does not present a problem in optical communications for a wavelength of input light around 1.55 μm.
FIGS. 7A through 7C are drawings that explain a method of compensating primarily second order dispersion. FIG. 7A explains wavelength vs. time characteristics and optical intensity vs. time characteristics, FIG. 7B explains a transmission example in which second order dispersion compensation is performed using a dispersion compensating fiber in a transmission path using SMF, while FIG. 7C explains a transmission example in a transmission path composed of only SMF.
In FIGS. 7A through 7C, reference symbols 501 and 511 are graphs showing the characteristics of signal light prior to being input into the transmission path, reference symbol 530 indicates a transmission path composed of SMF 531, reference symbols 502 and 512 are graphs showing the characteristics of a signal light when the signal light having the characteristics shown in graphs 501 and 511 is transmitted along transmission path 530 and comes out from transmission path 530, reference symbol 520 is a transmission path composed of dispersion compensating fiber 521 and SMF 522, and reference symbols 503 and 513 are graphs showing the characteristics of a signal light when the signal light having the characteristics shown in graphs 501 and 511 is transmitted along transmission path 520 and comes out from transmission path 520. Reference symbols 504 and 514 are graphs showing the characteristics of a signal light when the signal light having the characteristics shown in graphs 501 and 511 is transmitted along transmission path 520, comes out from transmission path 520, and then subjected to the desirable third order dispersion compensation described later according to the present invention, and closely coincide with graphs 501 and 511. In the graphs 501, 502, 503, and 504, each graph has wavelength plotted on the vertical axis and time (or actual time) plotted on the horizontal axis, while in the graphs 511, 512, 513, and 514, each graph has optical intensity plotted on the vertical axis and time (or actual time) plotted on the horizontal axis. Furthermore, reference symbols 524 and 534 indicate transmitters, while reference symbols 525 and 535 indicate receivers.
As was previously described, since in the case of conventional SMF, dispersion increases as a wavelength of a signal light becomes longer from 1.3 μm to 1.8 μm, during high speed communications or long distance transmissions, a delay occurs in the group velocity caused by dispersion. In transmission path 530 composed of an SMF, the signal light is delayed considerably at longer wavelengths more than at shorter wavelengths during transmission, and becomes as shown in graphs 502 and 512. The signal light that is deformed in this way may be unable to be accurately received as a signal light as a result of being unable to be distinguished from the signal light before and after it in, for example, high speed communications or long distance transmissions.
In the past, in order to solve such problems, dispersion was compensated (or corrected) by using, for example, a dispersion compensating fiber as shown in FIG. 7B. Dispersion compensating fiber of the prior art is made so that dispersion decreases as the wavelength becomes longer from 1.3 μm to 1.8 μm as previously described in order to solve the problem of SMF in which dispersion increased as the wavelength becomes longer from 1.3 μm to 1.8 μm. As shown with transmission path 520 of FIG. 7B for example, dispersion compensating fiber can be used by connecting dispersion compensating fiber 521 to SMF 522. In the above-mentioned transmission path 520, since the signal light is considerably delayed at longer wavelengths as compared with shorter wavelengths in SMF 522, and is then considerably delayed at shorter wavelengths as compared with longer wavelengths in dispersion compensating fiber 521, as shown in graphs 503 and 513, the grade of deformation can be held to a lower level than the deformation indicated in graphs 502 and 512.
However, in a compensation method for the second order wavelength dispersion of the prior art described above that uses a dispersion compensating fiber, dispersion compensation of signal light that has been transmitted along a transmission path cannot be performed in the state of the signal light prior to being input into the transmission path, namely until the shape of graph 501, and that compensation is limited to until the shape of graph 503. As shown in graph 503, in the compensation method for the second order wavelength dispersion of the prior art that uses a dispersion compensating fiber, light having a center wavelength of the signal light is not delayed in comparison with light having a shorter wavelength than the center wavelength of the signal light or light having a longer wavelength than the center wavelength of the signal light, while only the light of components having a shorter wavelength or longer wavelength than the light of the center wavelength component of the signal light is delayed. As shown in graph 513, sometimes ripple may occur in a part of the graph.
Furthermore, result of researches by the inventors of the present invention revealed that above-mentioned dispersion occurs in a signal light transmitting through not only optical fiber but also optical component such as wavelength selection filter. In the case of using such optical component for optical communications or optical equipment, the dispersion also occurs in a signal light.
These phenomena are becoming serious problems including the prevention of accurate signal reception accompanying greater needs for longer transmission distances and faster communication speeds of optical communications. For example, in the case of high speed communications in which signals are transmitted at a communications bit rate of 40 Gbps (40 gigabits per second) over a distance of 10,000 km or higher speed communications in which signals are transmitted at 80 Gbps over a distance on the order of several thousands km, these phenomena are a cause of considerable concern and are viewed as extremely serious problems. In such high speed communications, the use of conventional optical fiber communication systems is considered to be difficult. These phenomena are also becoming a serious problem from an economic standpoint of system construction, for example, such as even resulting in a need to change the material of the optical fibers themselves.
Since it is difficult to compensate for previously mentioned dispersion by the second order dispersion compensation only, and the third order or more dispersion compensation becomes necessary.
In the past, although DSF was used as an optical fiber (hereinafter, also to simply be referred to as a fiber) that reduce the second order dispersion for signal light having a wavelength around 1.55 μm, as is clear from the previously mentioned characteristics of FIG. 7A and FIG. 8, this fiber is not able to compensate the third order dispersion that is an object of the present invention.
In the realization of higher communication speeds and longer communication distances of optical communications, there is a growing awareness that the third order dispersion presents a significant problem, and its compensation is becoming an important topic. Although some attempts have been made to solve the problem of compensation of the third order dispersion, a third order dispersion compensating element or compensation method capable of adequately solving the problems of the prior art is not realized yet.
Although an example of using a fiber formed a diffraction grating pattern has been reported as a method for compensating the third order dispersion, this method has fatal shortcomings such as being not able to achieve the necessary compensation, having large loss, and having a large geometry. Moreover, the fiber is expensive and cannot be expected to be used practically.
As an example of optical dispersion compensating element for the above-mentioned third order dispersion compensation, the inventors of the present invention, independently from the present invention, succeeded in compensation of the third order dispersion to a certain extent by using an optical dispersion compensating element having a multi-layer film of a dielectric substance and so forth, and it brought a great advance of the optical communication technology of the prior art. However, as a request from the realization of higher communication speeds and longer communication distances of optical communications, in order to ideally perform the third order dispersion compensation in the case of high speed communications at a communication bit rate of 40 Gbps or 80 Gbps and so forth, or to adequately perform the third order dispersion compensation in multi-channel optical communications, realization of a dispersion compensating element or dispersion compensation method is desired that is able to adequately compensate the second order and the third order dispersion over an even broader wavelength band.
Since, at the present time, the dispersion that occurs in a signal light transmitting through a transmission path using optical fibers becomes a serious problem, the problem of optical component such as wavelength selection filter is not becoming an important topic. However, as described above, according to the results of research by the inventors of the present invention, the dispersion of the optical component is an important topic for higher communication speeds and longer communication distances of optical communications, and improvement of function and abilities of the optical component. At the present time, the optical component such as wavelength selection filter which is compensated the dispersion from such point of view is not sold.
In consideration of these points, the purpose of the present invention is to provide optical components, which is used for an optical communication systems in which an optical fiber is used for an optical communication path or optical equipments, sufficiently compensated the dispersion that occurs in a signal light and to provide its dispersion compensation method.